The present invention relates to the field of hydrodynamic bearing assemblies of the type which provides support and rotation for a high speed spindle element or hub. More specifically, the invention relates to improved hydrodynamic bearing assembly design which facilitates ease of assembly, as well as an improved method for filling a hydrodynamic bearing with fluid.
Disc drive memory systems have been used in computers for many years for storage of digital information. Information is recorded on concentric memory tracks of a magnetic disc medium, the actual information being stored in the form of magnetic transitions within the medium. The discs themselves are rotatably mounted on a spindle, the information being accessed by means of transducers located on a pivoting arm which moves radially over the surface of the disc. The read/write heads or transducers must be accurately aligned with the storage tracks on the disc to ensure proper reading and writing of information; thus the discs must be rotationally stable.
During operation, the discs are rotated at very high speeds within an enclosed housing by means of an electric motor which is generally located inside the hub or below the discs. One type of motor in common use is known as an in-hub or in-spindle motor. Such in-spindle motors typically have a spindle mounted by means of two ball bearing systems to a motor shaft disposed in the center of the hub. One of the bearings is typically located near the top of the spindle, and the other near the bottom. These bearings allow for rotational movement between the shaft and hub, while maintaining accurate alignment of the spindle to the shaft. The bearings themselves are normally lubricated by grease or oil.
The conventional bearing system described above, however, is prone to several shortcomings. First is the problem of vibration generated by the balls rolling on the raceways. Ball bearings used in hard disc drive spindles run under conditions that generally guarantee physical contact between raceway and ball, in spite of the lubrication layer provided by the bearing oil or grease. Hence, bearing balls running on the generally smooth but microscopically uneven and rough raceways, transmit this surface structure as well as their imperfection in sphericity in the form of vibration to the rotating disc. This vibration results in misalignment between the data tracks and the read/write transducer, limiting the data track density and the overall performance of the disc drive system.
Another problem is related to the application of hard disc drives in portable computer equipment and resulting requirements in shock resistance. Shocks create relative acceleration between the discs and the drive casting which in turn show up as a force across the bearing system. Since the contact surfaces in ball bearings are very small, the resulting contact pressures may exceed the yield strength of the bearing material, and leave long term deformation and damage to the raceway and the balls of the ball bearing.
Moreover, mechanical bearings are not easily scaleable to smaller dimensions. This is a significant drawback since the tendency in the disc drive industry has been to continually shrink the physical dimensions of the disc drive unit.
As an alternative to conventional ball bearing spindle systems, researchers have concentrated much of their efforts on developing a hydrodynamic bearing. In these types of systems, lubricating fluidxe2x80x94either gas or liquidxe2x80x94functions as the actual bearing surface between a stationary base or housing in the rotating spindle or rotating hub of the motor. For example, liquid lubricants comprising oil, more complex ferro-magnetic fluids or even air have been utilized in hydrodynamic bearing systems. The reason for the popularity of the use of air is the importance of avoiding the outgassing of contaminants into the sealed area of the head/disc housing. However, air does not provide the lubricating qualities of oil. The relative high viscosity of oil allows for larger bearing gaps and therefore grater tolerances to achieve similar dynamic performance.
In the case of a hydrodynamic bearing employing a liquid lubricant, the lubricating liquid must be reliably loaded into the bearing in order to maximize the load bearing capacity of the bearing. It is especially important to avoid the presence of any air bubbles within the oil carrying region of the bearing. With maximum oil fill in the hydrodynamic bearing, a stiffer motor is created.
The absence of air bubbles will minimize the pressure build-up inside the motor due to drops in ambient pressure and/or thermal expansion from increased temperature. This is due to the fact that while air bubbles will expand with changes in pressure or temperature, oil has little change in volume with such changes.
As little as 10% air in a typical fluid bearing could theoretically cause leakage through the seals at the ends of the bearings if the bearing is being stored at 0 degree C. Such failure of the bearing to contain the lubricant would cause contaminants to be expelled into the head disc region of the disc drive. The loss of some bearing fluid could cause the physical surfaces of the spindle and housing to contact one another, leading to increased wear and eventual failure of the bearing system.
Known techniques for filling the hydrodynamic bearing with oil require that the motor be capped after filling, i.e., that the entry channel for injecting the oil or other fluid into the bearing be closed and sealed after the oil is inserted. This is a difficult and complex process which easily results in the entrapment of air.
In view of the many long term benefits of a reliable hydrodynamic bearing design, the establishment of a reliable process for injecting fluid into the hydrodynamic bearing without allowing the entrapment of any air is highly desirable.
More particularly, a conventional single plate fluid dynamic bearing motor, comprises a shaft with a thrust plate supported at an end thereof. The process of filling the hydrodynamic bearings which support the shaft and thrust plate for relative rotation requires filling the hydrodynamic bearing with fluid, and then laser welding or insertion of a gasket to seal the gap between the sleeve which surrounds the shaft and the counterplate which faces the thrust plate to prevent oil leakage from a bearing. In such a design, the oil fill process is typically done in two steps: 1) In a first step, air is evacuated from the bearing cavity; and 2) In the second step, oil is filled into the cavity. An example of such a process appears in U.S. Pat. No. 5,601,125 assigned to the assignee of the present invention which demonstrates placing the assembled bearing into a sealed environment, establishing a vacuum, inserting the oil into the hydrodynamic bearing cavity or gap, and then completing the enclosure of the bearing gap to retain the fluid therein. This process, as well as the more direct processes of oil filling followed by laser welding or gasket insertion, required sophisticated and expensive oil filling machinery, and laser welding or other alignment steps.
Therefore, development of a design and an assembly method for simplifying insertion of oil into a fluid dynamic bearing remains highly desirable.
It is therefore an objective of the present invention to overcome the limitations of the prior art.
It is a further objective of the present invention to provide an improved design for a single plate fluid dynamic bearing motor which may be more easily filled with fluid.
A related objective of the invention is to provide a method for simplified filling of a single thrust plate fluid dynamic bearing motor. A further objective of the invention is to provide a method and apparatus for achieving a single plate fluid dynamic bearing motor in which the number of steps of assembly are diminished.
A related objective of the invention is to provide a method and apparatus for assembly of a single plate fluid dynamic bearing motor wherein the likelihood of air being trapped in the fluid is diminished, and steps for evacuation of air from the fluid probably do not need to be taken.
These and other objectives of the present invention are providing by assembling the shaft and thrust plate to be used in the fluid dynamic bearing assembly, and at least partially inserting the combined shaft and thrust plate into a sleeve which is adapted to support a hub which in turn may support one or more disks for rotation in a disc drive or similar environment. A counterplate cup of a generally U shaped or cup shaped design including a base which is substantially parallel to a top surface of the thrust plate, and sides of a diameter which is slightly greater than the diameter of the thrust plate and which are parallel to the thrust plate is provided, filled with oil, and having its open end facing the thrust plate.
Next the shaft and hub are pressed with the counterplate cup, the counterplate cup being pressed down over the end of the thrust plate so that the upraised sides of the cup fit easily over the ends of the thrust plate and can be pressed fit tightly over the upraised outer surface of the sleeve, tightly fitting the inside of the counterplate cup over the outside surface of the sleeve. During this press fit, the oil is pushed out of the region between the thrust plate upper surface and the counterplate cup bottom surface, and as the upraised sides of the cup fit over the outside of the sleeve, the oil fills the section between the lower surface of the thrust plate and the upper surface of the sleeve as well as the gap between the shaft and the surrounding sleeve. As a part of or subsequent to the step of pressing the counterplate cup and the shaft plate combination together, the thrust plate is also pressed down against the facing axial surface of the shaft until only a small gap remains so that the thrust bearing between the thrust plate and the sleeve is established. The end surface of the side of the cup is then sealed to the surface of the sleeve using an epoxy or the like so that the fluid cannot be lost during use.
The method and apparatus described summarily above and in detail below may be used either with a fluid dynamic bearing or a motor incorporating such a bearing, and is useful with bearings and motors of several designs. The method and apparatus are explained below with reference to a single motor model with which it is known to be useful. However, the scope of the present invention is not limited to its useablization with the single bearing design or motor model.
Other features and advantages of the present invention will become apparent to a person who skilled in the art who studies the following detailed description of a preferred embodiment of the method and apparatus of the present invention, given in conjunction with the accompanying drawings.